pkt-line: add PACKET_READ_GENTLE_ON_READ_ERROR option

Introduce PACKET_READ_GENTLE_ON_READ_ERROR option to help libify the
packet readers.

So far, the (possibly indirect) callers of `get_packet_data()` can ask
that function to return an error instead of `die()`ing upon end-of-file.
However, random read errors will still cause the process to die.

So let's introduce an explicit option to tell the packet reader
machinery to please be nice and only return an error on read errors.

This change prepares pkt-line for use by long-running daemon processes.
Such processes should be able to serve multiple concurrent clients and
and survive random IO errors.  If there is an error on one connection,
a daemon should be able to drop that connection and continue serving
existing and future connections.

This ability will be used by a Git-aware "Builtin FSMonitor" feature
in a later patch series.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pkt-line.c

@@ -306,15 +306,20 @@ static int get_packet_data(int fd, char **src_buf, size_t *src_size,
 		*src_size -= ret;
 	} else {
 		ret = read_in_full(fd, dst, size);
-		if (ret < 0)
+		if (ret < 0) {
+			if (options & PACKET_READ_GENTLE_ON_READ_ERROR)
+				return error_errno(_("read error"));
 			die_errno(_("read error"));
 		}
+	}
 
 	/* And complain if we didn't get enough bytes to satisfy the read. */
 	if (ret != size) {
 		if (options & PACKET_READ_GENTLE_ON_EOF)
 			return -1;
 
+		if (options & PACKET_READ_GENTLE_ON_READ_ERROR)
+			return error(_("the remote end hung up unexpectedly"));
 		die(_("the remote end hung up unexpectedly"));
 	}
 
@@ -343,6 +348,9 @@ enum packet_read_status packet_read_with_status(int fd, char **src_buffer,
 	len = packet_length(linelen);
 
 	if (len < 0) {
+		if (options & PACKET_READ_GENTLE_ON_READ_ERROR)
+			return error(_("protocol error: bad line length "
+				       "character: %.4s"), linelen);
 		die(_("protocol error: bad line length character: %.4s"), linelen);
 	} else if (!len) {
 		packet_trace("0000", 4, 0);
@@ -357,12 +365,19 @@ enum packet_read_status packet_read_with_status(int fd, char **src_buffer,
 		*pktlen = 0;
 		return PACKET_READ_RESPONSE_END;
 	} else if (len < 4) {
+		if (options & PACKET_READ_GENTLE_ON_READ_ERROR)
+			return error(_("protocol error: bad line length %d"),
+				     len);
 		die(_("protocol error: bad line length %d"), len);
 	}
 
 	len -= 4;
-	if ((unsigned)len >= size)
+	if ((unsigned)len >= size) {
+		if (options & PACKET_READ_GENTLE_ON_READ_ERROR)
+			return error(_("protocol error: bad line length %d"),
+				     len);
 		die(_("protocol error: bad line length %d"), len);
+	}
 
 	if (get_packet_data(fd, src_buffer, src_len, buffer, len, options) < 0) {
 		*pktlen = -1;

pkt-line.h

@@ -68,10 +68,15 @@ int write_packetized_from_buf_no_flush(const char *src_in, size_t len, int fd_ou
  *
  * If options contains PACKET_READ_DIE_ON_ERR_PACKET, it dies when it sees an
  * ERR packet.
+ *
+ * If options contains PACKET_READ_GENTLE_ON_READ_ERROR, we will not die
+ * on read errors, but instead return -1.  However, we may still die on an
+ * ERR packet (if requested).
  */
 #define PACKET_READ_GENTLE_ON_EOF        (1u<<0)
 #define PACKET_READ_CHOMP_NEWLINE        (1u<<1)
 #define PACKET_READ_DIE_ON_ERR_PACKET    (1u<<2)
+#define PACKET_READ_GENTLE_ON_READ_ERROR (1u<<3)
 int packet_read(int fd, char **src_buffer, size_t *src_len, char
 		*buffer, unsigned size, int options);